Extreme Water Levels8443970 Boston, MA

The monthly extreme water levels include a Mean Sea Level (MSL) trend of 2.63
millimeters/year with a 95% confidence interval of +/- 0.18 millimeters/year based on monthly MSL data from 1921 to 2006 which is equivalent to a change of 0.86 feet in 100 years.

The plots show the monthly highest and lowest water levels with the 1%, 10%, 50%, and 99% annual exceedance probability levels in red, orange, green, and blue. The plotted values are in meters relative to the Mean Higher High Water (MHHW) or Mean Lower Low Water (MLLW) datums established by CO-OPS (1 foot = 0.3 meters). On average, the 1% level (red) will be exceeded in only one year per century, the 10% level (orange) will be exceeded in ten years per century, and the 50% level (green) will be exceeded in fifty years per century. The 99% level (blue) will be exceeded in all but one year per century, although it could be exceeded more than once in other years.

A listing of the Top Ten Highest Water Levels at 110 long-term stations is also available as a table in meters or a table in feet above MHHW. No adjustment has been made for the rates of sea level rise or fall at each station. Therefore, stations with falling sea level trends are likely to have most of their Top Ten values near the beginning of their periods of record. An inferred level indicates that missing data at the peak water level were filled in. A high water mark is a physical mark near the station that can indicate the maximum elevation of a storm event. For some stations, older data recorded at a particular location may be stored in our database under a different station ID number.